Abstract:

An apparatus for treating a fluid to be injected into a subterranean
hydrocarbon-bearing formation includes a filtration unit having one or
more filtration membranes including either or both ultra-filtration
membranes and microfiltration membranes, and an ionic species removal
plant, wherein fluid to be injected is first treated by the filtration
unit and then treated by the ionic species removal plant. In a preferred
embodiment, the ionic species removal plant is a sulfate removal plant
which incorporates nano-filtration membranes. A method of treating fluid
to be injected into a subterranean formation is also disclosed. A
cleaning system and method for use in an apparatus for treating injection
fluid is also disclosed.

Claims:

1. A method of treating fluid to be injected into a subterranean
hydrocarbon-bearing formation, said method comprising the steps
of:flowing injection fluid through a filtration unit comprising at least
one filtration membrane including at least one of an ultra-filtration
membrane and a micro-filtration membrane; and thendriving said injection
fluid through an ionic species removal plant.

2. A method as claimed in claim 1, wherein the ionic species removal plant
is a sulfate removal plant for removing divalent sulfate ions from the
injection fluid.

3. A method as claimed in claim 1, wherein the ionic species removal plant
comprises at least one nano-filtration membrane.

4. A method as claimed in claim 1, further comprising the step of flowing
the injection fluid through a pre-filtration unit prior to flowing the
fluid through the filtration unit.

5. A method as claimed in claim 1, further comprising the step of flowing
the fluid through a deaerator.

6. A method of cleaning a fluid treatment apparatus incorporating a
plurality of filtration units each comprising at least one filtration
membrane including at least one of an ultra-filtration membrane and a
micro-filtration membrane, and an ionic species removal plant coupled to
and located downstream of the filtration unit, said method comprising the
steps of:providing a cleaning fluid supply taken from the fluid treatment
apparatus at a location downstream of the filtration unit; anddirectly
diverting at least a portion of the cleaning fluid supply from the fluid
treatment apparatus through the at least one filtration membrane.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application is a divisional of U.S. patent application Ser. No.
10/559,022 filed Nov. 30, 2005, which is the U.S. national phase of
PCT/GB2004/002310 filed Jun. 1, 2004, which claims priority of Great
Britain Patent Application GB 0312394.0 filed May 30, 2003.

FIELD OF THE INVENTION

[0002]The present invention relates to an apparatus and method for
treating an injection fluid, and in particular, but not exclusively, to
an apparatus and method for filtering and treating water to be injected
into a subterranean hydrocarbon-bearing formation.

BACKGROUND OF THE INVENTION

[0003]Extracting hydrocarbons from a subterranean formation involves
flowing hydrocarbons from the formation to surface through a production
well bore. In the early stages of production, the hydrocarbons are driven
into the production well and flowed to surface by pressure within the
formation. However, over time the formation pressure reduces until
natural extraction can no longer be sustained, at which stage some form
of artificial or assisted extraction is required. One common form of
artificial extraction involves the injection of a fluid medium into the
depleting formation through an injection well bore which extends from
surface in order to displace the hydrocarbons from the formation.
Conventionally, the fluid medium is aqueous and may be produced water or
sea water or the like. Fluid injection in this manner may also be
utilised as a form of matrix support in order to prevent collapse of the
reservoir after the hydrocarbons have been removed.

[0004]Where water injection is utilised to displace hydrocarbons from the
formation, or provide matrix support, it is important that the injection
water is compatible with the formation chemistry and is substantially
free from suspended or dissolved particles and colloidal and
macromolecular matter. This is required to prevent or at least minimise
plugging of the formation and associated wells, which occurs when
precipitates or suspended particles or the like accumulate and block, or
plug, fluid passageways. Such fluid passageways may include pores,
fractures, cracks or the like in the hydrocarbon-bearing rock formation,
or passageways defined by production and injection well bores. This
plugging can significantly reduce hydrocarbon production and in severe
cases can terminate production altogether.

[0005]In order to ensure that the injection fluid or water is
substantially free from suspended or dissolved particles and the like, it
is known in the art to treat the water prior to injection into the
formation. Treatment normally includes a combination of chemical and
mechanical or physical processes. For example, coagulants or flocculants
may be added to the water to encourage flocculation where heavy particles
or flocculus, known as "floc", are formed. The floe may then be removed
by sedimentation and/or by filtration whereby mechanical straining
removes a proportion of the particles by trapping them in the filter
medium. Conventional filtration apparatus for use in treating injection
water to remove such particulate material include multimedia filters
which consist of two or more layers of different or graded granular
material such as gravel, sand and anthracite, for example. The fluid or
water to be treated is passed through the filter and any suspended or
dissolved particles or the like will be retained in the interstices
between the granules of the different layers. It is therefore required
that the filter media be regularly cleaned to maintain a sufficient
filtration efficiency. Cleaning is conventionally achieved by a process
known as backwashing wherein clean or filtered water is passed through
the filter media in a reverse direction in order to dislodge the
particles which have been captured by the granules of the filter. It is
common to continually collect filtered fluid or water during normal
operation of the filtration apparatus, and when backwashing is required,
the apparatus is shut off and the collected filtered water is washed or
pumped using dedicated pump means in a reverse direction through the
filter apparatus. In conventional systems, around 100 to 150 m3 of
fluid may be required to be collected and stored prior to cleaning, which
utilises a considerable amount of valuable plant space, particularly on
off-shore platforms. This backwashing process, while effective, results
in the wastage of a large volume of treated water and the loss of a
portion of the filter media and also requires energy to operate the pump
means. Furthermore, in order to achieve adequate filtration, a large
quantity of filter media must be utilised which results in an extremely
large and heavy filtration unit requiring a considerable amount of
dedicated plant space which is at a premium on off-shore production
platforms, for example. Additionally, such multimedia filters require
considerable personnel attention to maintain, clean and replace the
filter media.

[0006]With regards to plugging caused by precipitate formation and
accumulation, this occurs when ionic species in the injection fluid or
water combines or reacts with compatible ionic species in water present
in the formation producing a precipitate or scale. For example, divalent
sulfate anions (SO42-) in the injection water will combine with
various cations which may be present in the formation water to form
substantially insoluble precipitates. For example, the formation water
may contain, among others: barium cations (Ba2+), which when
combined with sulfate produces a barium-sulfate or barite precipitate;
strontium cations (Sr2+) resulting in the formation of a
strontium-sulfate precipitate; or calcium cations (Ca2+) resulting
in the formation of a calcium-sulfate or anhydrite precipitate or scale.
As noted above, these resultant precipitates are substantially insoluble,
particularly barite, making any precipitate purging and removal/squeezing
process extremely difficult, complicated and expensive.

[0007]Additionally, the presence of sulfate in the injection fluid or
water provides a source of sulfur which thermophilic sulfate reducing
bacteria (SRB) present in the formation feed on, producing
hydrogen-sulfide (H2S) which causes souring of the well.
Hydrogen-sulfide is extremely corrosive and specialised equipment must be
used to accommodate the "sour" hydrocarbons, both at the
extraction/production stage and at the processing stage. Using injection
water with a high sulfate content can therefore sour an originally
"sweet" well.

[0008]Various methods have been proposed to provide a preventative
solution by removing the problematic, or precursor divalent ions from the
injection water before injection into the formation. For example, prior
art reference U.S. Pat. No. 4,723,603 assigned to Marathon Oil Company
discloses a process in which a feed water is treated to remove precursor
ions by a process of reverse osmosis to produce a treated injection water
product. The reverse osmosis technique involves forcing the feed water
through a semi-permeable reverse osmosis membrane under a pressure
greater than the osmotic pressure for the feed conditions and the
membrane type. It is known in the art that operational pressures of
reverse osmosis plants may be in the range of 50 to 70 barg. Thus,
considerable energy may be expended in operating the reverse osmosis
process where a significant flow rate of treated injection water is
required.

[0009]The reverse osmosis process is effective in removing ionic species
dissolved in an aqueous solution, but the efficiency and performance of
the process can depend heavily on the quality of the feed water to be
treated. For example, feed water which contains large quantities of
suspended solids or colloidal matter will cause fouling of the reverse
osmosis membrane, thus reducing the overall efficiency of the ionic
species removal process. It is therefore common to pre-treat the feed
water using conventional multimedia filters as discussed above.

[0010]It is among objects of embodiments of the present invention to
obviate or at least mitigate problems associated with prior art methods
of treating a fluid for injection into a hydrocarbon bearing formation.

SUMMARY OF THE INVENTION

[0011]According to a first aspect of the present invention, there is
provided an apparatus for treating a fluid to be injected into a
subterranean hydrocarbon-bearing formation, said apparatus comprising:

[0012]a filtration unit having a fluid inlet and a first fluid outlet,
said fluid inlet and first fluid outlet being in fluid communication via
a fluid passage;

[0013]at least one filtration membrane located within said fluid passage
such that the fluid inlet and first fluid outlet are in fluid
communication through the at least one filtration membrane, wherein said
at least one membrane includes at least one of an ultra-filtration
membrane and a micro-filtration membrane; and

[0014]an ionic species removal plant coupled to the first fluid outlet and
being in fluid communication therewith.

[0015]Thus, a fluid to be injected into a subterranean hydrocarbon-bearing
formation may be flowed through the filtration unit and through the at
least one filtration membrane such that any colloids, flocculants,
particulates and high molecular mass soluble species and the like will be
retained by the membrane by a mechanism of size exclusion to concentrate,
fraction or filter dissolved or suspended species within the fluid. This
filtered fluid is then be flowed to the ionic species removal plant to be
further treated. Thus, by locating the ionic species removal plant
downstream of the filtration unit of the apparatus of the present
invention, fouling of the ionic species removal plant by particles and
colloids and the like is substantially reduced. This arrangement improves
the performance of the ionic species removal plant and also reduces the
amount of cleaning required which conventionally involves the use of
potent chemicals which require safe disposal when spent.

[0016]When compared with traditional filtering methods used for treating
injection fluid, the apparatus of the present invention requires much
reduced plant space in that the filtration unit comprising either
ultra-filtration or micro-filtration membranes has a considerably smaller
footprint than multimedia filter units. For example, a filtration unit of
the apparatus of the present invention may define a footprint of 15 to 16
m2 whereas conventional multimedia filtration units for a similar
flux range may define a footprint of around 49 to 50 m2.
Furthermore, the quality of filtered fluid using the filtration unit of
the apparatus of the present invention would be considerably better than
that produced by multimedia filtration, in that a larger range of
particulate matter will be removed from the fluid being treated. For
example, the filtration unit of the present invention may be capable of
filtering particulate material as small as 0.005 to 0.1 microns, which
additionally may eliminate the requirement to introduce coagulants or
flocculants into the fluid prior to filtration. On the other hand,
conventional multimedia filtration units may fail to capture particles
such as silt having a nominal diameter of around 0.45 microns.
Accordingly, the quality of the fluid being fed to the ionic species
removal plant will be considerably better than that which would be
provided by conventional multimedia filtration units. Following from
this, fouling of the ionic species removal plant will be reduced
providing a corresponding reduction in the frequency of required plant
cleaning, cleaning coat and downtime, while extending plant operational
life. Moreover, the filtration unit of the present invention may be
easily adapted to be used in an automated treatment system, which
multimedia filters are generally not suitable for due to frequently
required personnel interaction. Numerous additional benefits of the
present invention in view of prior art or conventional systems exist,
some of which are considered below.

[0017]Preferably, the ionic species removal plant is a selective ionic
species removal plant such that only selected or required ions are
removed from a fluid requiring to be treated. Preferably, the ionic
species removal plant is a sulfate removal plant. Preferably, the ionic
species removal plant comprises at least one and preferably a plurality
of nano-filtration membranes adapted to reject divalent sulfate anions
(SO42-) while allowing monovalent ions to pass therethrough.
The nano-filtration membranes may permit ions such as sodium ions,
chlorine ions and potassium ions, for example, to pass therethrough,
wherein such monovalent ions may have a beneficial effect on the
formation by stabilising clays and the like. Accordingly, the ionic
species removal plant preferably does not utilise reverse osmosis.
Advantageously, the nano-filtration membranes also assist to remove
particles having nominal diameters of as low as 0.1 nanometres.

[0018]Preferably, the ionic species removal plant is pressure driven such
that fluid to be treated is driven under pressure therethrough. In a
preferred embodiment, the fluid is driven by a positive pressure
differential. Alternatively, the fluid may be driven by a negative
pressure differential by drawing a vacuum across the ionic species
removal plant. The pressure differential may be provided by pump means,
located either upstream or downstream of the ionic species removal plant,
depending on whether a positive or negative pressure differential is
required.

[0019]Advantageously, a pressure differential less than the plant osmotic
pressure for the feed conditions and membrane type within the ionic
species plant is utilised. For example, a pressure differential of
between 5 to 40 bar may be utilised to drive the fluid to be treated
through the ionic species removal plant. Accordingly, achieving ionic
species removal at pressures lower than that required for reverse osmosis
results in a more energy efficient treatment process.

[0020]Advantageously, the fluid inlet of the filtration unit is adapted to
be coupled to a fluid source for fluid communication therewith. The fluid
source may be seawater, for example, or water or brine produced from a
subterranean formation or the like.

[0021]The fluid inlet of the filtration unit of the apparatus may be
adapted to be coupled to the fluid source via a pre-filtration unit.
Preferably, the pre-filtration unit comprises strainers having sieve
sizes of between 80 to 150 microns. Thus, the fluid intended to be fed to
the apparatus of the present invention may be pre-filtered in order to
remove larger suspended particles and the like which may block or foul
the at least one membrane located within the filtration unit.

[0022]Advantageously, the at least one filtration membrane defines a
plurality of pores each having a nominal diameter or equivalent dimension
of between 0.005 to 0.1 microns for ultra-filtration membranes and 0.05
to 2 microns for micro-filtration membranes. Preferably, the molecular
weight cut-off of an ultra-filtration membrane for use in the filtration
unit is between 1,000 and 500,000.

[0023]In one embodiment of the present invention, the at least one
membrane may comprise a ceramic material. Alternatively, the at least one
membrane may comprise a polymeric material. For example, a crystalline
polymeric material may be utilised where micro-filtration membranes are
required. Additionally, amorphous polymers may be utilised where
ultra-filtration membranes are required. Suitable polymeric membrane
materials include PVDF, polypropylene, polysulfone, cellulosic and other
proprietary formulations.

[0024]Preferably, the filtration unit comprises a pressure vessel.

[0025]Preferably, the apparatus includes a plurality of membranes arranged
within the filtration unit. The membranes may consist entirely of
ultra-filtration membranes, or entirely of micro-filtration membranes, or
a combination thereof. Advantageously, the number of membranes required
for the filtration unit may be selected in accordance with the available
size of the filtration unit and the required filtration surface area to
achieve the desired fluid fluxes. For example, the filtration unit may
comprise between 60 and 80 membranes.

[0026]In one embodiment of the present invention, the membrane utilised in
the filtration unit is of a tube configuration which consists of a porous
support tube having a membrane material cast on an inside wall thereof.
In this embodiment, fluid to be treated may be caused to pass radially
outwardly through the membrane material. Alternatively, fluid may be
caused to pass radially inwardly through the membrane material.

[0027]In an alternative embodiment of the present invention, the membrane
utilised in the filtration unit may be of a plate and frame
configuration. In this configuration, a flat sheet membrane is secured in
a plate and frame unit to form a membrane screen. This arrangement is
particularly advantageous in that virtually any membrane may be cut to
the appropriate shape and secured or installed in the unit.

[0028]In a further alternative embodiment, the membrane may be provided in
a spiral wound configuration. In this embodiment, conventionally, a
membrane laminate is provided which consists of two substantially flat
membrane sheets or layers, separated by a filtrate carrier. Three sides
of the laminate are sealed to envelop the filtrate carrier within the
membrane sheets, with a fourth side of the laminate being secured,
longitudinally, to a perforated tube. The laminate is then rolled around
the perforated tube, with the outwardly facing surfaces of the membrane
sheets being separated by a screen or corrugated spacer, to produce a
substantially cylindrical cassette. In this arrangement, fluid to be
treated is flowed into the perforated tube and is caused to pass radially
outwardly through the spirally wound membranes of the cylindrical
cassette. Alternatively, fluid to be treated may be passed radially
inwardly through the spirally wound membranes.

[0029]In a preferred embodiment of the present invention, the membrane may
be provided in a hollow fibre configuration. In this embodiment, a
plurality of membranes are preferably provided. This hollow fibre
configuration comprises a plurality of elongate hollow or tubular fibres
of a suitable membrane material, longitudinally aligned within the
filtration unit. The hollow fibres are similar in form to the tube
configuration membranes, with the exception that no porous support tube
is required. In this preferred embodiment, fluid to be treated is flowed
along the inside of the membranes and is caused to pass radially
outwardly through the membrane material, generally referred to as an
"in-to-out" configuration. Alternatively, the fluid to be treated may be
flowed along the outer surface of the membranes and caused to pass
radially inwardly through the membrane material, generally referred to as
an "out-to-in" configuration.

[0030]Depending on the service mode of the filtration membrane, as
discussed below, the filtration unit may comprise a second fluid outlet
to provide an exit for unfiltered fluid or fluid used in a backwashing
cleaning operation. It should be appreciated that any unfiltered fluid
will likely have a higher concentration of particulates, colloids and
suspended matter and the like than the feed fluid, as the solid matter
retained by the at least one membrane will be entrained into the stream
of fluid and directed and flowed towards the second fluid outlet. Thus,
the feed fluid entering the filtration unit via the fluid inlet will be
separated into two fluid streams, the first being filtered fluid driven
through the at least one membrane and exiting through the first fluid
outlet, and the second being unfiltered or concentrated fluid exiting
through the second fluid outlet. The provision of the second fluid outlet
and thus second flow path assists in cleaning the at least one filtration
membrane, reducing the amount of backwashing required and maintaining a
reasonably high filtration efficiency.

[0031]Advantageously, the filtration unit of the present invention may
operate in either a dead-end flow service mode or a cross-flow service
mode. In dead-end flow mode of operation (also known as direct flow)
fluid is forced perpendicularly, directly through the at least one
membrane. Accordingly, there is only a feed flow entering the filtration
unit via the fluid inlet, and a filtrate flow exiting the filtration unit
via the first fluid outlet. The dead-end flow approach typically allows
for optimal recovery of feed water in the 95 to 98% range, but is
typically limited to feed streams of low suspended solids (typically
<10 NTU turbidity). With dead-end flow a depth of particle build up is
formed on the surface of the at least one membrane.

[0032]In cross-flow mode, fluid passes parallel to the at least one
filtration membrane, often at a velocity an order of magnitude higher
than the velocity of the fluid stream passing through the membrane. With
this operation, three flow paths are established, the first being feed
fluid entering the filtration unit via the fluid inlet, the second being
filtered fluid exiting the filtration unit via the first fluid exit, and
the third being unfiltered or concentrated fluid exiting the filtration
unit via the second fluid outlet. The flow of unfiltered fluid through
the second fluid outlet assist in cleaning the at least one membrane by
constantly removing filtered material which would otherwise accumulate on
the surface thereof. Accordingly, the cross-flow mode is typically used
for feed fluids with higher suspended solids (typically 10 to 100 NTU
turbidity). The cross-flow mode of operation typically results in 90 to
95% recovery of feed fluid. This is a significant improvement over
conventional multimedia filtration units which may provide a fluid
recovery in the region of 80%.

[0033]Preferably, the apparatus of the present invention comprises a
plurality of filtration units. Advantageously, the filtration units are
arranged in parallel. That is, the fluid inlet of each filtration unit
may be coupled to a single fluid feed inlet stream, and the first fluid
outlet of each filtration unit may be coupled to single first fluid feed
outlet stream. Additionally, where provided, the second fluid outlet of
each filtration unit may be coupled to a single second fluid outlet
stream. Advantageously, each filtration unit may be adapted to be
individually isolated in order to permit selective cleaning, replacement
or repair or the like without requiring complete shutdown of the
apparatus. In an alternative embodiment, the filtration units may be
arranged in series such that filtration is achieved in a staged process.

[0034]In one embodiment of the present invention, eight pairs of
filtration units may be provided.

[0035]Preferably, means are provided for creating a pressure differential
between the fluid inlet and the first fluid outlet of the filtration unit
such that fluid to be treated is pressure driven through the at least one
filtration membrane. Advantageously, the pressure differential may be
provided by pump means, located either upstream or downstream of the
filtration unit, depending on the required pressure gradient. In one
embodiment, the fluid may be driven by a positive pressure differential.
This positive pressure differential may be achieved using a pump means
located upstream of the filtration unit. Advantageously, the filtration
unit and associated at least one membrane may be adapted to operate at a
positive pressure differential of at least 2 barg, with an upper pressure
limit being restricted by, for example, the design limitation of the
specific type of membrane being utilised.

[0036]Alternatively, the fluid may be driven by a negative pressure
differential by drawing a vacuum across the filtration unit. This
negative pressure differential may be achieved utilising a pump means
located downstream of the filtration unit. Advantageously, the filtration
unit and associated at least one membrane may be adapted to operate at a
negative pressure differential of, for example, between 0.07 and 0.55
barg. In this embodiment, the at least one membrane may be submerged
within a tank or vessel at atmospheric pressure, with the negative
pressure differential across the at least one membrane providing a
driving force to drive the fluid from the tank or vessel through the
membrane.

[0037]Preferably, where a plurality of filtration units are provided, a
single pump means may be provided to create a pressure differential
across the membranes. Alternatively, individual pump means may be
provided.

[0038]Preferably, the apparatus of the present invention further comprises
a cleaning system for use in cleaning at least the at least one membrane
of the filtration unit. The cleaning system is preferably adapted to
operate while the at least one membrane remains located within the
filtration unit, which is conventionally referred to as
"cleaning-in-place". Advantageously, the cleaning system utilises a
portion of fluid from the fluid outlet of the filtration unit.
Preferably, the cleaning system is particularly adapted for use where a
plurality of filtration units are provided. In this embodiment, a
filtration unit requiring to be cleaned may be isolated from the
remaining units such that fluid to be filtered cannot pass. Therethrough.
Accordingly, once isolated, the filtration unit may be backwashed by
forcing fluid taken from the fluid outlet of the operational filtration
units in a reverse direction through the at least one membrane located
therein. The filtration units may be selectively isolated by isolating
means such as valves or the like.

[0039]Advantageously, the filtration units are adapted to accommodate the
overall required filtration flux or fluid rate when one or more of the
filtration units are isolated. Accordingly, fluid taken from the fluid
outlet of the operational filtration units is preferably fed directly to
the isolated unit to be cleaned. Thus, the requirement to continually
collect fluid in a separate storage tank to subsequently be used for
cleaning is eliminated, as is the requirement to shut down the entire
filtration unit to accommodate cleaning. That is, the operational
filtration units will be capable of producing sufficient filtered fluid
for a portion to be piped or fed directly into the cleaning system and
used to backwash an isolated filtration unit without requiring additional
pump means. However, embodiments of the invention may comprise additional
or supplementary pump means.

[0040]In one embodiment, fluid for use in cleaning may be taken directly
from the first fluid outlet of the filtration unit or units such that
fluid used for cleaning has not been treated by the ionic species removal
plant. Alternatively, fluid for use in the cleaning system may be taken
from an outlet of the ionic species removal plant. Accordingly, in both
embodiments, the fluid used in the cleaning system is at least subjected
to filtration by the filtration unit.

[0041]Advantageously, the fluid utilised in the cleaning system may be
used in a cleaning process to clean the ionic species removal plant.

[0042]Preferably, the cleaning system further comprises a chemical
cleaning system. Conveniently, the chemical cleaning system, in use,
requires a chemical solution to be driven across the filtration unit
and/or the ionic species removal plant in a normal flow direction.
Advantageously, the chemical solution may comprise fluid extracted from
the first fluid outlet of the filtration unit, or alternatively may
comprise fluid extracted from an outlet of the ionic species removal
plant, with the required chemical or chemicals added thereto. Preferably,
the chemical cleaning system comprises a pump means to drive the chemical
solution across one or both the filtration unit and ionic species removal
plant.

[0043]Advantageously, the chemical cleaning system may be utilised, for
example, once every month during normal operation to clean the filtration
unit, whereas the chemical cleaning system may be utilised once every two
to three month during normal operation, for example, to clean the ionic
species removal plant. It should be noted that chemical cleaning of the
ionic species removal plant is required less frequently than the
filtration unit due to the fact that the filtration unit of the present
invention supplies high quality feed water to the ionic species removal
plant.

[0044]Preferably, the cleaning system further comprises air cleaning means
wherein compressed air is driven through one or both the filtration unit
and ionic species removal plant.

[0045]Advantageously, the apparatus further comprises a deaerator in order
to remove oxygen and other gasses from the fluid in order to prevent
aerobic bacteria growth during an injection process. In one embodiment,
the deaerator may be located downstream of the ionic species removal
plant. Alternatively, the deaerator may be located upstream of the
filtration unit or alternatively further may be located between the
filtration unit and the ionic species removal plant.

[0046]Preferably, the filtration unit of the apparatus operates with a
nominal flux of litres of treated product fluid per meter square of
filtration membrane per hour of at least 201/m2/h. More preferably,
the filtration unit operates with a nominal flux of between 80 to
2001/m2/h.

[0047]Preferably also, the operating pH of the fluid may be adjusted
within the range 1 to 13. More preferably, the operating pH range is 6.5
to 8.5 depending on the membrane material used.

[0048]According to a second aspect of the present invention, there is
provided a method of treating fluid to be injected into a subterranean
hydrocarbon-bearing formation, said method comprising the steps of:

[0049]flowing injection fluid through a filtration unit comprising at
least one filtration membrane being at least one of an ultra-filtration
membrane and a micro-filtration membrane; and then

[0050]driving said injection fluid through an ionic species removal plant.

[0051]Preferably, the ionic species removal plant is a sulfate removal
plant for removing divalent sulfate ions from the injection fluid.
Preferably also, the ionic species removal plant comprises at least one
nano-filtration membrane.

[0052]Advantageously, the method further involves the step of flowing the
injection fluid through a pre-filtration unit prior to flowing the fluid
through the filtration unit.

[0053]Beneficially, the method may further include the step of flowing the
fluid through a deaerator.

[0054]According to a third aspect of the present invention, there is
provided an injection system for injecting fluid into a subterranean
hydrocarbon-bearing formation, said system comprising:

[0055]a filtration unit comprising at least one filtration membrane being
at least one of an ultra-filtration membrane and a micro-filtration
membrane;

[0056]an ionic species removal plant coupled to an outlet of the
filtration unit; and

[0057]injection pump means coupled to the ionic species removal plant and
adapted for pressurising fluid from the ionic species removal plant to be
injected into a hydrocarbon-bearing formation.

[0058]Preferably, the ionic species removal plant is a sulfate removal
plant.

[0059]According to a fourth aspect of the present invention, there is
provided a cleaning system for use in a fluid treatment apparatus
incorporating a plurality of filtration units each comprising at least
one filtration membrane being at least one of an ultra-filtration
membrane and a micro-filtration membrane, and an ionic species removal
plant coupled to and located downstream of the filtration unit, wherein
the cleaning system comprises:

[0060]a cleaning fluid supply taken from the fluid treatment apparatus at
a location downstream of the filtration unit; and

[0061]means for diverting at least a portion of the cleaning fluid supply
directly from the fluid treatment apparatus through the at least one
filtration membrane.

[0062]Preferably, the cleaning fluid is diverted in a reverse direction
through the at least one filtration membrane to effect backwashing
thereof.

[0063]Preferably, the diverting means is a pipe network communicating
fluid from a location downstream of the filtration unit to the at least
one filtration unit.

[0064]Preferably also, the cleaning fluid is driven through the at least
one membrane by pump means associated with the fluid treatment apparatus.
Accordingly, the cleaning system preferably does not comprise separate
pump means.

[0065]In one embodiment of the present invention, the cleaning fluid
supply may be taken from the fluid treatment apparatus at a location
upstream of the ionic species removal plant. Alternatively, the cleaning
fluid supply may be taken at a location downstream of the ionic species
removal plant. In either of the alternative embodiments, the cleaning
fluid is at least filtered fluid treated by the filtration unit.

[0066]Preferably, the cleaning system further comprises fluid isolation
means for selectively preventing cleaning fluid taken from the fluid
treatment apparatus being passed through the at least one membrane. The
isolation means may be valve means such as a manually operated or
motorised valve.

[0067]Advantageously, the fluid treatment apparatus comprises a plurality
of filtration units, and the cleaning system comprises filtration unit
isolation means to selectively isolate a filtration unit requiring to be
cleaned using the cleaning fluid supply.

[0068]Preferably, the cleaning system comprises means for diverting at
least apportion of the cleaning fluid supply directly from the fluid
treatment apparatus through the ionic species removal plant. Preferably
also, the cleaning fluid is directed through the ionic species removal
plant in a reverse direction to effect backwashing thereof.

[0069]According to a fifth aspect of the present invention, there is
provided a method of cleaning a fluid treatment apparatus incorporating a
plurality of filtration units each comprising at least one filtration
membrane being at least one of an ultra-filtration membrane and a
micro-filtration membrane, and an ionic species removal plant coupled to
and located downstream of the filtration unit, said method comprising the
steps of:

[0070]providing a cleaning fluid supply taken from the fluid treatment
apparatus at a location downstream of the filtration unit; and

[0071]directly diverting at least a portion of the cleaning fluid supply
from the fluid treatment apparatus through the at least one filtration
membrane.

[0072]Preferably, the cleaning fluid is directly diverted through the at
least one filtration membrane in a reverse direction to effect
backwashing thereof.

[0073]Preferably, the method further comprises the step of directly
diverting at least a portion of the cleaning fluid through the ionic
species removal plant, preferably in a reverse direction to effect
backwashing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0074]These and other aspects of the present invention will now be
described, by way of example only, with reference to the accompanying
drawings, in which:

[0075]FIGS. 1 and 2 are diagrammatic representations of alternative
embodiments of an apparatus for treating water to be injected into a
hydrocarbon-bearing formation according to the present invention;

[0076]FIG. 3 is a diagrammatic representation of a sulfate removal process
using a nano-filtration membrane;

[0077]FIG. 4 is a diagrammatic representation of apparatus for treating
water to be injected into a hydrocarbon-bearing formation in accordance
with an alternative embodiment of the present invention;

[0078]FIG. 5 is a perspective view of a portion of an apparatus for
treating injection water in accordance with an embodiment of the present
invention;

[0079]FIG. 6 is a diagrammatic representation of a backwashing system for
use in an apparatus for treating an injection water in accordance with an
embodiment of the present invention; and

[0080]FIG. 7 is a diagrammatic representation of a chemical cleaning
system for use in an apparatus for treating an injection water in
accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0081]Referring initially to FIG. 1, there is shown a diagrammatic
representation of a water treatment apparatus or system 10 in accordance
with an embodiment of the present invention. The system includes a drive
pump 12, a filtration unit 14, a sulfate removal plant 16 and an
injection pump 18. The filtration unit 14 includes a fluid inlet 20 and a
first fluid outlet 22, between which fluid inlet 20 and first fluid
outlet 22 is located a bank of filtration membranes 24. In the embodiment
shown, the bank of membranes 24 is composed of ultra-filtration membranes
which define pores having nominal diameters or equivalent dimensions of
between 0.005 to 0.1 microns with a molecular cut-off weight of between,
for example, 1,000 to 500,000. In an alternative embodiment, the bank of
membranes may be composed of micro-filtration membranes which define
pores having nominal diameters or equivalent dimensions of between 0.05
to 2 microns.

[0082]The sulfate removal plant 16 includes a fluid inlet 26 and a first
fluid outlet 28 and a bank of nanofiltration membranes 30 located
therebetween. As shown in FIG. 3, which is a diagrammatic representation
of a surface 32 of a nano-filtration membrane of the bank of membranes
30, divalent sulfate anions (SO42-) 34 are rejected at the
surface 32 of the nano-filtration membrane, partly due to size exclusion
and partly due to repulsion caused by a negative charge on the surface 32
of the membrane. As shown, however, the membrane allows the passage of
monovalent anions such as chlorine anions (Cl.sup.-) therethrough. The
passage of such ions is preferred as they may assist to stabilise
formation clays and the like once injected with the water into the
formation.

[0083]In use, feed water such as sea water from a fluid source (not shown)
is pressurised by the drive pump 12 to the required pressure determined
by, among other things, membrane type and the filtrate backpressure
required. The pressure may be, for example, selected to be between 2 and
5 barg, and possibly greater. In the embodiment shown in FIG. 1,
therefore, the fluid is driven across the filtration unit 14 by a
positive pressure differential. The fluid is pressure driven into the
inlet 20 of the filtration unit 14 and is forced through the bank of
membranes 24 such that any colloids, flocculants, particulates and high
molecular mass soluble species and the like will be retained by the
membranes 24 by a mechanism of size exclusion to concentrate, fraction or
filter dissolved or suspended species within the water. As shown, the
filtration unit 14 includes a second fluid outlet 38 through which
unfiltered water may exit carrying the particles and colloids and the
like retained by the bank of membranes. This operation mode is termed
cross-flow mode and assists to wash or continually clean the membranes 24
to minimise fouling.

[0084]Upon exiting the filtration unit 24 through the first fluid outlet
22, the filtered water passes through the sulfate removal plant wherein
the membranes 30 reject sulfate anions, as shown in FIG. 3. The sulfate
removal plant 16 includes a second fluid outlet 40 through which high
sulfate concentrated water is rejected from the apparatus 10. Although
not shown, the sulfate removal plant may be associated with a separate
drive pump to pressurise the fluid from the filtration unit to the
required pressure for sulfate removal. In this regard, the fluid passing
through the sulfate removal plant 16 is of a pressure which is below the
osmotic pressure for the feed conditions and membrane 30 type.
Accordingly, the sulfate removal plant 16 does not operate by reverse
osmosis.

[0085]Water from the first fluid outlet of the sulfate removal plant 16 is
then pressurised by the injection pump 18 and is injected into a
depleting hydrocarbon-bearing formation via a cased injection well bore
42.

[0086]An air release valve 43 is provided at a location downstream of the
sulfate removal plant 16 and filtration unit 24, wherein the air valve 43
may be opened when the apparatus is initially put into operation to allow
air to be displaced or bled from the system. Additionally, the air valve
43 may be utilised to allow air used in an air cleaning process to be
vented from the apparatus 10.

[0087]An alternative embodiment to that shown in FIG. 1 is shown in FIG.
2, in which like components share like reference numerals, incremented by
100. As shown, the apparatus 110 includes a drive pump 112, a filtration
unit 114 including a bank of ultra-filtration membranes 124, a sulfate
removal plant 116, and an injection pump 118. However, in this
embodiment, the drive pump is located downstream of the filtration unit
114 such that water from a water source (not shown) may be driven through
the filtration unit 114 by a negative pressure differential. For example,
a vacuum pressure of between 0.07 and 0.55 barg may be drawn across the
bank of membranes 124.

[0088]A further alternative embodiment to that shown in FIG. 1 is shown in
FIG. 4, in which like components share like reference numerals,
incremented by 200. The apparatus 210 of FIG. 4 comprises a filtration
unit 214 having a bank of membranes 224, a sulfate removal plant 216
including a bank of sulfate removing nano-filtration membranes 230, and
an injection pump 218. The apparatus 210 additionally includes a
pre-filtration unit 202 located upstream of the filtration unit 214, and
which comprises strainers having sieve sizes of between 80 to 150
microns. Thus, the water intended to be fed to the filtration unit 214 is
pre-filtered in order to remove larger suspended particles which may
block or foul the bank of membranes 224 located within the filtration
unit 114.

[0089]Reference is now made to FIG. 5 in which there is shown a portion of
another alternative embodiment of the apparatus 10 of FIG. 1.
Accordingly, like components share like reference numerals, incremented
by 300. The fluid treatment apparatus 310 of FIG. 5 comprises a plurality
of filtration units 314 arranged in a rack, generally represented by
reference numeral 350. Each filtration unit 314 includes a fluid inlet
320 and a first fluid outlet 322. Additionally, each filtration unit
encloses a bank of filtration membranes (not shown), which in the
embodiment shown are of a hollow fibre form. The portion of the apparatus
310 shown in FIG. 5 further includes a fluid inlet stream or manifold 352
to which the fluid inlet 320 of each filtration unit 314 is connected,
and a fluid outlet stream or manifold 354 to which the first fluid outlet
322 of each filtration unit 314 is connected. In his way, the filtration
units are considered to be connected in parallel, such that one or more
individual units 314 may be independently isolated with a valve (not
shown), without shutting off the entire apparatus 310. This arrangement
therefore permits individual units to be cleaned, for example by
backwashing as discussed below, while the apparatus 310 remains
operational.

[0090]Reference is now made to FIG. 6 in which there is shown a
diagrammatic representation of an injection water treatment apparatus 410
incorporating a backwashing system in accordance with an embodiment of
the present invention. The apparatus of FIG. 6 is similar to that shown
in FIG. 1 and as such like components share like reference numerals,
incremented by 400. The apparatus 410 comprises a drive pump 412, a
plurality of filtration units 414a, 414b (only two shown) connected in
parallel, and a sulfate removal plant 416. Each filtration unit 414a,414b
includes a bank of membranes 424, and the sulfate removal plant 416
includes a bank of nano-filtration membranes 430. Each filtration unit
comprises isolation valves 460,462 which permit a respective unit
414a,414b to be independently isolated.

[0091]In use, fluid is pressurised by the pump 412 and is fed to a fluid
inlet 420 of each operational filtration unit 414a,414b, wherein the
fluid is forced through the membranes 424 and then exits through
respective first fluid outlets 422. Each filtration unit 414a,414b
includes a second fluid outlet 438 through which unfiltered or cross-flow
fluid exits the respective units 414a,414b. The filtered fluid is then
driven along a fluid conduit 464 towards the sulfate removal plant 416.

[0092]The backwashing system of the apparatus 410 comprises a fluid path
466 extending between the fluid conduit 464 and one filtration unit 414a,
and another fluid path 468 extending between the fluid conduit 464 and
another filtration unit 414b. It should be noted that a fluid path
extending from the fluid conduit 464 to each filtration unit is provided.
As shown, fluid path 466 incorporates a valve 470, and fluid path 468
incorporates a valve 472.

[0093]Assuming backwashing of filtration unit 414a is required, isolating
valves 460 are first closed such that fluid no longer passes from the
fluid inlet 420 to the first fluid outlet 422. It should be noted that
the remaining filtration units, including unit 414b are adapted to
accommodate an increased flux when the filtration unit 414a is isolated
in this manner, in order to maintain a uniform output. Following this,
valve 470 is opened in order to tap a portion of the fluid flowing along
conduit 464, which fluid is flowed along path 466 to filtration unit
414a. This fluid is then driven by the system pressure achieved by the
drive pump 12 through the bank of membranes 424 in unit 414a in a reverse
direction in order to effect backwashing. The backwashing fluid is dumped
from filtration unit 414a through the second fluid outlet 438. Once
sufficient backwashing has been achieved, valve 470 may be closed and
valves 460 may be opened to bring the filtration unit 414a back into
operation.

[0094]The backwashing system of the present invention is particularly
advantageous in that it does not require the entire system to be
shutdown. Additionally, because the backwashing system is operated by
drive pump 412, no additional pump means is required. Furthermore,
because fluid is taken directly from fluid conduit 464, there is no
requirement for storage tanks or the like to store fluid to be used in a
backwashing process.

[0095]Reference is now made to FIG. 7 in which there is shown a
diagrammatic representation of an injection water treatment apparatus 510
incorporating a chemical cleaning system in accordance with an embodiment
of the present invention. The apparatus of FIG. 7 is similar to that
shown in FIG. 1 and as such like components share like reference
numerals, incremented by 500.

[0096]The apparatus 510 comprises a drive pump 512, a filtration unit 514
and a sulfate removal plant 516. A fluid tap 580 is provided between the
filtration unit 514 and the sulfate removal plant 516 in order to tap a
portion of filtered fluid from the filtration unit 514. This tapped fluid
is then collected in a suitable tank 582 and any required chemicals 584
are added to create a chemical solution 581. When chemical cleaning is
required, the filtration unit 514 is isolated from the fluid source (not
shown) with valve 586 and the chemical solution 581 is driven via pump
588 to the fluid inlet 520 of the filtration unit.

[0097]It will be understood by a person of skill in the art that the
embodiments hereinbefore described are merely exemplary of the present
invention and that various modifications may be made thereto without
departing from the scope of the invention. For example, each of the
embodiments shown in FIGS. 1, 2 and 4 to 7 may further comprise a
deaerator in order to remove oxygen and other gases from the fluid being
treated. Additionally, in each of the embodiments shown, the sulfate
removal plant my comprise separate pump means. Furthermore, in the
apparatus 410 of FIG. 6, fluid is tapped to be used for backwashing at a
location upstream of the sulfate removal plant 416. However, fluid may be
tapped from a location downstream of the sulfate removal plant 416.
Similarly, fluid tap 580 in the embodiment shown in FIG. 7 may be located
downstream of the sulfate removal plant 516. Additionally, in the
embodiments shown in FIGS. 4, 6 and 7, fluid may alternatively be driven
by a negative pressure differential across the filtration unit.